The cause of the sound is the water flowing on both sides of a faucet washer, o-ring or a shut off washer.
Water flows on both sides of the faucet washer; the washer vibrates and makes a sound similar to a clarinet in a woodwind orchestra, and resonates that sound throughout the water supply pipes in your whole house. This sound can raise quite a racket.
The problem is at one of your shut-off valves or in the toilet itself, maybe even the valve at the refrigerator. If you have this occurrence at many different locations you may have a lot of dirt, grit or mineral deposits that flow through your pipes and make short-life of your valve seats by getting this grit behind the valve washers that make this sound. Once the grit is behind the washer it will continue to hum or whistle when there is enough flow just like a clarinet woodwind instrument.
I suggest you look for a common valve or shut-off that these locations share, maybe even the entry valve for the whole house water supply. Also consider that many of these valves are the same age and may be creating the same sound at several locations, same whistle but at many places.
The solution is to find the sound and change the faucet washer in the shut-off or to change to whole shut-off valve. Another option is to change the toilet ball-cock which is the toilet flushing guts in the toilet tank.
My first suggestion is to change the guts in the toilet because it is easiest, and next change the water shut-offs. There may be two or three shut-offs up-line to your toilet all the way from the main house shut-off, valve in the basement to the bath or bathrooms above, as well as the shut-off nearest the fixture.
This problem can happen on faucets, toilets and the whole house. It is most often a shut-off valve but finding the humming valve can be a challenge. It may be several valves.